Lithium secondary batteries are broadly used for portable electronic devices, personal computers and the like. Whereas the size reduction and weight reduction are demanded, raising the energy density is an important problem.
Conceivable methods of raising the energy density of lithium secondary batteries include some methods, and among these, raising the operating voltage of the batteries is effective. In lithium secondary batteries using lithium cobaltate or lithium manganate as their positive electrode active material, the average operating voltage is 3.6 to 3.8 V (4-V class) with respect to a metallic lithium reference. This is because the operating voltage is determined by the redox reaction (Co3+Co4+ or Mn3+Mn4+) of cobalt ions or manganese ions.
On the other hand, since a spinel compound in which in lithium manganate, part of manganese is substituted with nickel or the like, for example, LiNi0.5Mn1.5O4, exhibits a potential plateau in the region of 4.5 V or higher, the use of such a spinel compound as a positive electrode active material enables the operating voltage of 5-V class to be realized. In a positive electrode using such a spinel compound, manganese is present in the 4-valence state, and the operating voltage of the battery is determined by the redox of Ni2+Ni4+ instead of the redox of Mn3+Mn4+.
The capacity of LiNi0.5Mn1.5O4 is 130 mAh/g or higher; the average operating voltage is 4.6 V or higher with respect to metallic lithium; and although the lithium absorbing capacity is lower than that of LiCoO2, the energy density is higher than that of LiCoO2. For these reasons, LiNi0.5Mn1.5O4 is promising as a positive electrode material.
In batteries using a high-potential positive electrode active material such as LiNi0.5Mn1.5O4, however, the operating voltage becomes higher than in batteries using LiCoO2, LiMn2O4 or the like as a positive electrode active material, and the decomposition reaction of an electrolyte at the contact portion of a positive electrode with the electrolyte is liable to progress. The decomposition reaction generates gas. Since the generation of the gas raises the internal pressure of cells and causes swelling of laminate cells, it is a problem on practical uses. Hence, there has been demanded an electrolyte with high voltage resistance, enabling the generation of such gases to be suppressed. Further the similar phenomenon becomes a problem also in conventionally used 4-V class batteries, for example in a storing condition under a high temperature environment for a long period. As electrolytes with high voltage resistance capable of suppressing the gas generation, fluorinated solvents and the like are thought of. Examples of the potential solvents include fluorinated carbonates, fluorinated carbonate esters, fluorine-containing ether compounds and fluorine-containing phosphate ester compounds, all of which are fluorinated solvents. Among these, fluorine-containing ether compounds are useful because of being high in the life improvement effect and being comparatively low in the viscosity.
For example, Patent Literature 1 states that in a lithium secondary battery containing a positive electrode active material operating at potentials of 4.5 V or higher, its nonaqueous electrolytic solvent contains a fluorine-containing phosphate ester compound. Further Patent Literature 2 describes a lithium ion secondary battery in which its nonaqueous electrolyte contains a fluorinated ether.